Rescaled Leptonic Unitarity Triangles and Rephasing Invariants

The field of neutrino physics has made significant progress in measuring the strength and frequency of neutrino and antineutrino oscillations in the past two decades. It is clear that the amplitudes involved in the neutrino oscillation probabilities are all rephaping invariants of the quartet forms of the elements of the PMNS mixing matrix. We show in this paper how these quartet observables can be directly linked to the rescaled leptonic unitarity triangles within the framework of three active neutrinos. We provide a systematic discussion of the nine CP-conserving quartets ${\cal R}^{}{γk} \equiv {\rm Re} \left [ V^{}{αi} V^{}{βj} V^{*}{αj} V^{}{βi}\right ] $ along with the universal Jarlskog invariant of CP violation ${\cal J} \equiv \sum_γε^{}{αβγ} \sum_k ε^{}{ijk} ; {\rm Im} \left [ V^{}{αi} V^{}_{βj} V^{}{αj} V^{*}{βi} \right ]$, and place particular emphasis on the matter effect on these quartets. In addition to the well-known Naumov relation for the Jarlskog invariant ${\cal J}$, similar relations connecting ${\cal R}$ in vacuum and its effective counterparts $\widetilde{\cal R}$ in matter are introduced and examined in detail. We find that the effective CP-conserving invariants $\widetilde{\cal R}^{}_{αi}$ in matter can be regarded as linear combinations of their vacuum counterparts. With the latest global fit data of neutrino masses and mixing elements, numerical analyses are carried out to give an intuitive understanding of how these rephasing invariants evolve as the matter density increases.

💡 Research Summary

The paper presents a comprehensive framework that links rephasing‑invariant quartets of the PMNS mixing matrix to rescaled leptonic unitarity triangles (UTs) in both vacuum and matter. Starting from the well‑known parametrisation of the 3 × 3 unitary matrix V by three mixing angles (θ₁₂, θ₂₃, θ₁₃) and a CP‑violating phase δ, the authors define nine CP‑conserving quartets
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